Recent trends in the access communications market show that data rates up to 100 Mb/s which are provided by VDSL systems using Vectoring as defined in ITU-T Recommendation G.993.5 are not sufficient for all applications and bit rates up to 1.0 Gb/s are required in some cases. To achieve these targets, for wire-based implementations currently copper pairs connecting the CPE must be as short as 50-100 m. Operation using so short loops requires installation of many small street/MDU (Multi Dwelling Unit) cabinets called Distribution Points (DP) that serve a very small number of customers, e.g. 16 or 24 and are connected to the backbone via fiber (fiber to the distribution point FTTdp).
Vectoring is used in systems operating from a DP [GC02], to reduce far-end crosstalk (FEXT), which is absolutely necessary to obtain high bit rates. To improve energy efficiency and to reduce hardware complexity, synchronized time division duplexing (S-TDD) is used for FTTdp instead of frequency division duplexing (FDD) which is used in VDSL.
DPs shall allow very flexible installation practices: they should be light and easy to install on a pole or house wall, or basement, without air-conditioning. The most challenging issue for these flexible connection plans is providing DPs with power. The only solution found is so-called “reverse feeding” when the equipment of the DP is fed by the connected customer. The requirement of reverse power feeding and the small size of the DP implies substantial restrictions on the power consumption of the DP, which main contributors are DSL transceivers. Further, the customer unit is often required battery-powered operation (to support life line POTS during power outages). The latter applies low power requirements also to DSL transceivers of the CP equipment.
Conventional DSL systems transmit data continuously on all lines sharing a cable binder. Whenever there is no data available, idle bytes are transmitted. With this type of static operation, the system stability and performance is maintained.
In current DSL systems (e.g., ADSL), low-power modes and data rate adaptation use a method that reduces bit loading and TX (Transmit) power on the line when data traffic turns to be slow and reconstructs it back when high speed traffic is back. Other proposed methods use called SRA (Seamless Rate Adaptation) to reconfigure the bit rate and TX power of the links. Both reconfiguration methods are too slow to perform adaptive link reconfiguration with respect to the actual traffic requirements of the subscribers.
Also in terms of power saving, current DSL transceivers only allow power saving by transmit power reduction. The transmit power in VDSL systems is in the range of 14 dBm to 20 dBm and therefore, the transmit power largely contributes to the overall power consumption.
However in FTTdp applications, the transmit power is only a small portion of the overall power consumption, because the aggregate transmit power is in the range of 4 dBm. Components like the analog and digital frontend electronics consume power irrespective of the transmit power, but these components significantly contribute to the overall power consumption, because they operate at much higher frequencies of 100 MHz or 200 MHz in comparison to 8 MHz-30 MHz in VDSL.
Therefore, to provide significant power savings also analog and digital components of the transceiver, such as analog front end (AFE) and digital front end (DFE), need to be switched into low-power (standby) state. This operation mode is called Discontinuous Operation.
In currently operated systems using Vectoring, such as ITU G.993.5, a time consuming procedure called “orderly leaving” is required before a link can be switched off. If a line is disconnected without orderly leaving, the remaining active lines of the binder experience substantial performance drops. Therefore, AFE and DFE cannot be turned off for short time which substantially reduces power savings.
Therefore, improvements in using low power modes like discontinuous operations together with vectoring would be desirable.